1. Field of the Invention
This invention relates to the recovery of caustic from a lignin free solution of hemicellulose and caustic by electrolysis.
2. Description of Related Prior Art
In the production of high purity cellulose fiber used to manufacture rayon, cellulose based films. etc., pulp processed by conventional kraft pulping processes and bleached using chlorine, chlorate, and hydrogen peroxide, the pulp is highly delignified and very clean. The bleaching process steps are very aggressive since low fiber strength and low lignin content so as to obtain high brightness is essential. By the time the pulp enters the last bleaching stages, the lignin content of the pulp is very low. During the last bleaching stages, the hemicellulose and other wood sugars are removed utilizing caustic extraction. Fresh caustic is fed to these stages at a concentration of about 30 to about 35 percent by weight. In a final washing stage, clean water is used to wash away the hemicellulose from the pulp. The water dilutes the caustic solution of hemicellulose to provide a dilute solution off hemicellulose and caustic having a concentration of about 1 to about 10 percent, preferably, about 6 percent by weight caustic. The dissolved hemicellulose gives this solution a brown color. In the paper mill, some of the hemicellulose caustic solution is evaporated to 35 percent caustic content by weight and recycled for use in other parts of the paper mill where the hemicellulose content of the caustic solution is not detrimental such as the initial pulp bleaching and extraction stages in the process.
Because the recovery system for recovering pulping chemicals often represents the critical production limitation in the kraft pulping process because of the limited capacity inherent in the high capital cost for such a recovery system, the capacity of the paper mill to process the entire hemicellulose caustic solution often is inadequate and, accordingly, other methods of recovering a caustic solution, preferably, free of hemicellulose, are needed.
In U.S. Pat. No. 5,061,343, a process is disclosed for the recovery of sodium hydroxide and other values from spent liquors and bleach plant effluents in a kraft pulping mill. This patent discloses a process for removing lignin from an aqueous alkaline liquid by a combination of electrolytic acidification of this liquid and chemical acidification.
U.S. Pat. No. 4.584,076 is cited in the above patent as disclosing a method of treating sulfur-free spent liquors in an electrolysis cell to recover lignin and sodium hydroxide.
It is considered that these references are not directly relevant prior art to the inventive process disclosed herein for the electrolytic recovery of sodium hydroxide and other values such as hemicellulose, oxygen and hydrogen utilizing an electrolytic cell to concentrate an aqueous solution of hemicellulose caustic so as to allow recycling of the sodium hydroxide contained therein.